Semi-chemical pulping process



United States Patent Q SEMI-CHEMICAL PULPING PROCESS Charles H. Davenport, El Dorado, Ark., and Burton E. Helberg, York, Pa assignors to Monsanto Chemical Company, St. Louis, Mo., a corporation of Delaware No Drawing. Application March 12, 1956 Serial No. 570,717

4 Claims. (Cl. 92-6) The present invention relates to an improved semichemical process for producing pulp from hardwoods or softwoods and mixtures thereof. More particularly, the invention relates to an ammonia base semi-chemical pulping process whereby various mixed woods may be pulped without requiring segregation of the different wood species commonly found intermingled in pulp forests, particularly in the southern regions of the United States.

In the true chemical pulping processes in commercial use, Wood chips are completely reduced to the required pulp by the action of pulping chemicals in a digester. The requirements for pulping chemicals are rather high, with a resulting low yield of pulp based on dry weight of the wood prior to digestion. The maximum pulp yields in most chemical pulping operations rarely exceeds 50%. A

relatively long digestion period is also required for producing good chemical pulp which reduces the throughput for a given piece of equipment.

To obviate to some extent the several serious drawbacks inherent in the true chemical pulping operation, the socalled semi-chemical pulping process came into commercial use. In this process wood chips are partially digested to a degree permitting them to be reduced to a pulp by mechanical action in a subsequent operation. The yield of pulp is much greater, often being in the range of 70-80% of the dry weight of the original wood. The digestion period is substantially reduced, thereby increasing the capacity of pulping equipment. that considerable lignin and other matter remains in the pulp in this process, it is difficult to bleach the resulting pulp, even with uneconomical amounts of chlorine, hence most semi-chemical pulp is used primarily in the manufacture of unbleached corrugated board and wrapping paper.

Semi-chemical pulping operations of present commercial importance employ a weak kraft solution or an acid sodium sulfite or neutral sodium sulfite cooking liquor. Neutral sulfite liquors comprising sodium sulfite with sodium bicarbonate to maintain neutrality during the cook have been used with success in the semi-chemical process on selected individual hardwoods, and the resulting pulp is reportedly bleachable. The sodium base semi-chemical pulping process, while commercially feasible, also suffers from several disadvantages. The cost of chemicals is relatively high, making recovery of spent chemicals a necessity. In order to economically recover the chemicals, the sodium base operation is usually tied in with a kraft paper mill. Where kraft facilities do not exist it is usually not economical to use the sodium base semichemical process. Furthermore, there have been recurring shortages of the sodium chemicals used in this process, necessitating costly shut downs or conversions of plant facilities. Furthermore, it is necessary to segre gate the wood according to species prior to pulping. In most forests, particularly in the southland, a number of different tree species will be found growing side by side. It is a. distinct disadvantage to have to separate, or selectively cut difierent species for pulping purposes.

Due to the fact 2,882,147 Patented Apr. 14, 1959 ICC The ammonia base semi-chemical pulping process, to which the present invention is directed, has been studied for a number of years, and has been introduced into commercial practice for short periods. To our knowledge however, prior to the present invention, the ammonia base semi-chemical pulping process was essentially experimental in nature. The problems of bleaching and chemical recovery, to mention a few, have presented rather formidable obstacles to the adoption of the process. Also, until recent years the supply situation and cost of ammonia had to be reckoned with. As with the sodium base process, prior attempts at ammonia base semi-chemical pulping have been generally restricted to selected hardwoods.

It is the principal object of the present invention to provide an improved semi-chemical pulping process and more particularly an ammonia base semi-chemical process.

Another object is to provide a semi-chemical process whereby different species of trees may be reduced to chips and pulped in the proportion found growing in the forest, obviating the necessity of segregating the woods according to type and species prior to pulping.

A further object is to provide an improved semi-chemical pulping process whereby the pulp, including that derived from pine, may be bleached with a minimum of bleaching agent.

A still further object is to provide a process whereby unbleached pulp may be produced with a reduced digestion time.

A further object is to provide a pulping process employing a minimum amount of sulfur value.

Further objects and advantages of the invention will be apparent from the description and examples which follow.

In accordance with the present invention, we have discovered that an excellent wood pulp may be produced from a hardwood, a softwood or mixtures thereof, by employing an ammonia base digestion liquor in the semichemical process, wherein the ammonia value is present in the required ratio of combined to free ammonia, and a certain range of temperature and pressure is employed. We have found that a good grade of unbleached pulp may be produced in about 2 hours versus the 4 hours required in the sodium base semi-chemical process. We have further found that a digestion period of 4-6 hours in our digestion liquor, followed by an aqueous treatment which may comprise steam or superheated water at a temperature not substantially above the digestion temperature in the case of hardwoods or mixtures thereof, renders a pulp which may be bleached to a high degree of whiteness with a minimum of expensive bleaching a cut.

To our knowledge, prior to our invention, pine had not been employed in the semi-chemical process, whether sodium or ammonia base. In accordance with our process, pine alone, or admixed with other coniferous or soft woods, or admixed up to 50% with hardwood chips, may be satisfactorily pulped, and bleached if desired. In the case of pine chips, either alone or admixed with other woods, we have found that a digestion period of as little as 4 hours is sufficient to soften the chips prior to reducing them to a fibrous pulp by mechanical action. We have also discovered that the resulting pulp may be readily bleached if it is subjected to a short treatment with weak nitric acid prior to treating with the bleaching agent.

According to our invention, semi-chemical hardwood pulps may also be easily bleached using slightly less chlorine than required in the sodium base semi-chemical process, after the post digestion aqueous treatment. Surprisingly, our post digestion treatment had no beneficial effect upon the bleachability of sodium base pulp in a number of test runs conducted for the purpose. The reason for this apparent anomaly is not understood. Furthermore, using steam or water for the leaching step at or near its atmospheric boiling point produces no beneficial change in the pulp produced by our process. Also the bleachability of pine is not improved by the aqueous treatment. With such wood, even when admixed with hardwoods, the nitric acid treatment described is required for good bleachability.

It should also be pointed out that the hot aqueous treatment or leaching step which we apply to hardwoods, while essential primarily for improved bleachability, performs an important ancillary function in that substantially all of the ammonia and sulfur values remaining in the pulp after draining off the digestion liquor as well as water soluble organics are removed in a relatively concentrated solution. This is of particular importance in connection with chemical recovery operations. Also by removing most of the digestion chemicals in this step, the amount of Water and time required in subsequent pulp washing and refining operations is greatly reduced, eliminating the need for inclusion of this water in the chemical recovery, and contributing further to the economics of our process.

The chemical recovery problem in connection with pulping operations has been previously mentioned. With most commercial pulping processes, recovery of the spent chemicals is essential for economic reasons, and hence comprises a major item of expense in the process. While we prefer to recover spent chemicals for re-use in our process, it is not economically essential to do so, since the sulfur value employed has been reduced to a minimum. If recovery of the digestion chemicals is desired, we prefer the method as set forth in US. Patent No 2,596,241.

In accordance with a preferred embodiment of the present invention, hardwood chips, with or without an initial pro-steaming, are digested in an ammoniacal ammonium sulfite liquor for 2-6 hours at a cooking temperature of l65170 C. and at 100125 p.s.i.g. pressure. The cooking liquor is then drained off, and the chips are given an aqueous treatment which may comprise steaming at a temperature of about 170 C. and a pressure of 100 p.s.i.g., or leached with superheated water at a temperature of about 170 C. After steaming or leaching the chips are blown from the reactor and reduced to a fibrous pulp in an attrition mill. The pulp is bleached by the three-stage bleaching procedure, more or less conventional in the art.

The liquor used in our process for cooking the pulp is a mixture of ammonium sulfite and ammonium hydroxide. The ammonia present with the sulfite we refer to as combined ammonia and that with the hydroxide as free ammonia. In the process of the present invention the ratio of combined ammonia to free ammonia is preferably in the range of 3:1 to 611. While we prefer using combined ammonia as ammonium sulfite, ammonium sulfide may be substituted.

The total amount of ammonia (free-I-combined) used in the cooking liquor is preferably about by weight of the oven-dry wood used. Higher amounts of total ammonia tend to reduce the cooking time, but require more extensive recovery from spent liquor and also cause higher cooking pressures at a given temperature. Less than about 10% ammonia calls for longer cooking time.

The total amount of cooking liquor employed according to the present invention is about 6:1 by weight of liquor to oven-dry wood. Thus for every ton of ovendry wood processed the cooking liquor contains 200 pounds of ammonia (171 pounds of which is combined as ammonium sulfite), 322 pounds of sulfur dioxide, and 11,278 pounds of water. Wide variations within the specified ranges in the concentration of the cooking liquor may be used without significant effects, the exact concentration depending somewhat on the digester design.

The cooking temperature and pressure preferred in the present invention are l70 C. and 100-125 p.s.i.g. respectively. The preferred pressure range is usually the autogenous pressure at the digestion temperature. Venting during cooking is not essential. However, if practiced, the vented gases should be passed through suitable ammonia recovery equipment and provision made to add back to the digester any ammonia so vented.

As previously indicated, the cooking time may be varied considerably depending on the type pulp desired and the kind of wood being pulped. Longer cooking times are preferred in making pulp to be bleached, whereg as shorter times are permissible for pulps not intended for bleaching. According to the present invention mixed hardwoods are digested in from 2 to 6 hours; good bleachable pulp requiring about 4 hours time. Soft woods, such as pine, are digested in from 3 to 6 hours; 4 hours being required to make a bleached pulp. The digestion times set forth above are exclusive of the time required to reach digestion temperature, and to drain 0E liquor at end of digestion.

The post digestion aqueous treatment is carried out in the digester after draining ofi the cooking liquor. When steam is used in this step, it is introduced at about 100 p.s.i.g. and a temperature of about -180 C.; any condensate which forms is preferably recycled over the cooked chips and withdrawn from the digester approximateiy every 15 minutes. Steaming is carried out for about 1 hour, and then the chips are blown out of the digester. Carrying out this step in 15 minute cycles for about an hour has given superior results in a minimum amount of time.

When hot-water leaching is used, 80-100 C. water is pumped into the digester, heated to about l70-l80 C., recycled for 15 minutes, and then withdrawn from the digester. This is repeated 3 times before blowing the chips from the reactor, giving a total treatment time of about 1 hour. As with the use of steam, this procedure has proven of superior value in improving the bleachability of the pulp.

After the wood chips have been cooked and steamed or hot-water leached, they are refined in an attrition mill and finally bleached, where a substantially white product is desired. The preferred bleaching technique is a 3-stage procedure using 13-15% total chlorine based on the ovendry unbleached pulp. The first stage uses chlorine water for 30 minutes with a 2% pulp consistency, 80% of the total chlorine being used. The second stage is a 30 minute caustic extraction and is carried out by adding caustic soda to the mixture from the first stage, the pH being 1111.5. Following the second stage the mixture is centrifuged, washed thoroughly and reduced to a thick slurry and then treated for 2 hours in a third stage, at a temperature of 35-55 C., to the action of the remainder of the chlorine, the chlorine being added as sodium hypochlorite. The pulp is then centrifuged and washedthoroughly. The above described bleaching procedure is more or less conventional in the pulping art.

The bleaching procedure described above is satisfactory for bleaching hardwood pulp made by the process of the present invention. However, when soft-wood pulp such as pine is to be bleached we have found it necessary, as heretofore indicated, to use a nitric acid treatment just prior to the first stage of the chlorine bleach procedure, with the hot aqueous treatment being omitted. Thus, pine pulp from the refining step is treated with 10% nitric acid solution at 60-70 C. prior to the conventional 3-stage bleaching procedure to produce bleached pine or softwood semi-chemical pulp.

The following examples are presented to better illustrate the mode of practicing the present invention. Conventional commercial type pulping equipment was employed in carrying out these examples. A 2 cu.,ft. digester, made by the Patterson-Kelley Company, was used for digesting the pulp, and a Sprout-Waldron Model 103-A refiner was used for refining the digested chips.

As indicated, the examples are presented for illustrative purposes, and are not intended to limit the scope of the invention. Modifications obvious to those skilled in the art are intended to fall within the spirit and scope of the present invention.

Example 1 A Patterson-Kelley digester was charged with 6000 grams of oven-dry mixed hardwood chips and 34,805 grams of ammoniacal ammonium sulfite liquor containing 600 grams NH and 967 grams S0 The mixed hardwood chips were comprised, by oven-dry weight, of 37.5% white oak, 37.5% red oak and 25% gum. The chip size was /2"-1" width and length by thickness. The temperature and pressure of the digester were brought to 160-165 C. and 100-120 p.s.i.g. in one hour and held at that point for 4 hours, after which the chips were blown from the digester into a cyclone separator. The blown chips were then refined in a Sprout-Waldron Model 103-A refiner to a freeness of 845 cc. (Schopper-Riegler freeness tester) and bleached according to the conventional 3-stage bleaching technique previously described. The bleached pulp had a 49 brightness (G.E. brightness tester), and was obtained in 56.7% yield based on the weight of ovendry wood charged to the digester. The unbleached pulp yield was 67.8%. The bursting or Mullen strength of a sample sheet prepared from this pulp was 41.2 pounds/sq. in. The tear strength per sheet was 76.5 grams (TAPPI Standard T220M-53, values for 800 cc. Schopper-Riegler freeness).

Example 2 This example was carried out exactly like Example 1 except that a 1 hour aqueous treatment was employed after the 4 hour digestion cycle. Thus, following the digestion cycle, the cooking liquor was drained out of the digester and steam was injected into the digester for a period of 1 hour. The steaming was carried out at 175- 180 C. and 120-130 p.s.i.g. Condensate forming in the digester was drained every 15 minutes, but the digester was not otherwise vented during the steaming period. Following the steaming, the chips were blown from the reactor, refined to 820 cc. freeness, and bleached. Substantially all of the ammonia and sulfur values were removed from the pulp in the steaming operation, reducing the number of washes in the refining step to a minimum. The bursting strength (Mullen) of a sample paper made from this pulp was 34.4 pounds/sq. in. The tear strength was 82.5 grams/sheet. The bleached pulp had a 78.5 brightness, and was obtained in 50.4% yield. The unbleached yield was 57.3%.

Comparing the brightness of the bleached pulp from Example 1 with Example 2 (49 vs. 78.5 respectively), it is seen that the aqueous treatment is a very efiective aid to bleaching. This effect is unexpected, since such steaming in the sodium-base semi-chemical process does not aid subsequent bleaching.

Example 3 This example was also carried out according to Example 1 except that a 1 hour hot water leaching period was employed after the 4 hour digestion period. Thus, following the digestion, the cooking liquor was drained out of the digester and hot water sufiicient to cover the chips was injected into the digester. The water was further heated to 170-175 C. at 100-120 p.s.i.g. and circulated through the digester for 15 minutes, then drained. A total of three such hot water leaches was made. Following the third leach the chips were blown from the digester, refined to 815 cc. freeness, and bleached. The bleached pulp had an 82.5 brightness, and was obtained in 51.9% yield. The unbleached pulp yield was 58.6%. The bursting strength (Mullen) of a sample paper from this pulp was 36.0 pounds/sq. in. The tear strength was 78.8 grams/sheet.

Comparing the brightness of the bleached pulp from Example 1 with Example 3 (49 versus 82.5 respectively), it is seen that the hot water treatment is equivalent to, and perhaps even more effective than, the steam treatment as an aid to bleaching. Carrying out the aqueous treatment at C. instead of 170-175 C. showed no significant etfect on pulp bleachability.

Example 4 In this example southern yellow pine chips were digested and processed in a manner similar to the procedure outlined in Example 2. The pine chips were cooked for 6 hours, steamed at 180-185" C. and p.s.i.g., blown and refined to 875 cc. freeness. After refining, the pulp was treated with 10% nitric acid solution for 1 hour at 50-60% C. The acid treated pulp was then water washed and bleached. The bleached pulp had a 63 brightness, and was obtained in 54.5% yield. The unbleached pulp yield was 70.7%. The bursting strength (Mullen) of a sample paper from this pulp was 50 pounds/sq. in. and the tear strength was 76.8 grams/ sheet.

When pulping pine according to the procedure outlined above in Example 4 but without the nitric acid treatment, the pulp is not bleachable. Thus without the nitric acid treatment pine pulps of only about 30 brightness are obtained. The steaming step was found to be unnecessary for pine and most softwoods in conjunction with the acid treatment.

Example 5 In this example a mixture of pine and hardwood chips was pulped. The mixture was made up of 50% southern yellow pine, 18%% white oak, 18%% red oak, and 12 /z% gum on an oven-dry basis. The chips were processed according to the procedure outlined in Example 4, except that the hot-water leaching technique described in Example 3 was substituted for the steaming step. After the hot water treatment, the chips were blown, refined to 840 cc. freeness, treated with 10% nitric acid and bleached. The bleached pulp had a 72 brightness, and was obtained in 52.1% yield. The unbleached yield was 63.5%.

The procedure according to Example 5 provides a method for making pulp from mixed woods-hard and soft. This is of definite economic significance. By this procedure pulp can be made from wood without regard to its identity, thus eliminating the necessity of selectively cutting trees in the forests. The hot water treatment was included primarily for the removal of the ammonia and sulfur values from the digested chips. No significant improvement in the bleachability of pine was noted from the aqueous treatment alone. The nitric acid was efliective on the hardwoods component as well as pine. This treatment is too harsh for pulp comprising all hardwoods however, as the pulp strength is adversely affected.

Where an unbleached pulp is desired for making corrugated boxes and the like, our process, as described in Examples 6 and 7 which follow, is particularly attractive in that cooking time is a minimum and yields at a maximum.

Example 6 Six thousand grams of gum wood chips (oven-dry basis) were charged to the digester in the manner of Example 2. The digestion liquor comprised at 6:1 ratio of combined to free ammonia diluted to a total volume of 35,400 grams by the addition of tap water.

The digester was brought up to a temperature of -165 C., and a pressure of 100-110 p.s.i.g. The chips were digested under these conditions for a period of two hours, after which they were blown out and removed "to the refiner for reduction to pulp by mechanical action.

An analysis of the spent liquor showed a pH of 6.73. All of the free ammonia was consumed during the digestion period and 22.8% of the combined ammonia.

The chips were refined to a freeness of 855 cc. (Schopper-Riegler). The yield of pulp was 73.6% based on oven-dry weight of chips.

The pulp, at the end of the digestion period, was suffi ciently softened to the extent that it could easily be shredded by hand; hence the refining time was reduced to a minimum. The pulp was lighter in color, as compared with similar pulp prepared by the sodium base process.

Example 7 A 50-50 mixture of red oak and white oak chips was charged to the digester in the manner of the preceding example, 6000 grams, based on oven-dry weight, being used.

The chips were digested for two hours as previously described, blown from the digester, and refined to a freeness of 800 cc. (Schopper-Riegler). Yield was 72.2%. The pH 'of the spent digestion liquor was 6.50. All of the free ammonia and 37.0% of the combined ammonia was consumed during the digestion.

The digested chips were soft and easily reduced to pulp in the refiner with a minimum of attrition. The pulp was of light color.

Thus, according to the present invention, semi-chemical pulp can be produced from softwoods, hardwoods or mixtures thereof. The digestion time required to make unbleached semi-chemical pulp by this invention is roughly only one-half that required for sodium base semichemical operations. The total digestion time for making bleachable pulp via the present invention is about the same as that required to make unbleached semi-chemical pulp via the sodium base process. A minimum amount of sulfur and ammonia value is used in the cooking liquor employed in the present invention, thus reducing the overall cost of chemical recovery in the process.

We claim:

1. -A semi-chemical process for producing a bleached wood pulp from mixed hardwoods which comprises digesting mixed hardwood chips in'a digestion liquor comprising free ammonia as ammonium hydroxide and combined ammonia as ammonium sulfite in the ratio of combined to free ammonia of 3:1 to 6:1, the total ammonia comprising about 10% by weight based on the oven-dry chips being digested, said digestion being carried out at a temperature of about 165-170 C., a pressure of pounds/sq. in. gauge, and for a period of about 4 hours, recovering ammonia and sulfur values from said digestion liquor, subjecting saidchips after said digestion period to an aqueous treatment at a temperature not substantially above said digestion temperature, refining said chips by mechanical action after said aqueous treatment until said chips are reduced to a fibrous pulp, and treating said pulp with a bleaching agent.

2. A process as claimed in claim 1 wherein said postdigestion aqueous treatment comprises steaming said digested chips at about 100 pounds/sq. in. pressure of about 1 hour at a temperature of about l80 C. whereby substantially all water-soluble organics and all of the ammonia and sulfur values remaining in said chips from the digestion thereof are removed and improved bleachability imparted thereto.

3. A process as claimed in claim 2 wherein said steam treatment is carried out in approximately fifteen minute cycles, with the condensate being removed between each cycle.

4. A process as claimed in claim 1 wherein said aqueous treatment comprises subjecting said digested chips to the action of water superheated to a temperature of about 170 C., said treatment being carried out in approximately 15 minute cycles for a period of about an hour whereby substantially all Water-soluble organics and all of the ammonia and sulfur values remaining in said chips from the digestion thereof are removed from said chips and improved bleachability imparted thereto.

References Cited in the file of this patent UNITED STATES PATENTS 1,016,178 Sammet Jan. 30, 1912 1,100,519 Brech June 16, 1914 1,891,337 Seaman Dec. 20, 1932 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 2,882,147 April 14, 1959 Charles' H. Davenport et a1.

It is hereby Certified that errOrpappeaI-s in the printed specification of the above numbered patent requiring cor- 1 IGCtiOll and that the said Letters Patent should read as Corrected below Column 7, line 24 Example 7 for "37.0%" read Signed and sealed this 20th day of September 1960.

? -V(SEAL) Attest:

.33 A NE ROBERT c. WATSON Attesting Officer v v Coaniinioner of Patents 

1. A SEMI-CHEMICAL PROCESS FOR PRODUCING A BLEACHED WOOD PULP FROM MIXED HARDWOODS WHICH COMPRISES DIGESTING MIXED HARDWOOD CHIPS IN A DIGESTION LIQUOR COMPRISING FREE AMMONIA AS AMMONIUM HYDROXIDE AND COMBINED AMMONIA AS AMMONIUM SULFITE IN THE RATIO OF COMBINED TO FREE AMMONIA OF 3:1 TO 6:1, THE TOTAL AMMONIA COMPRISING ABOUT 10% BY WEIGHT BASED ON THE OVEN-DRY CHIPS BEING DIGESTED, SAID DIGESTION BEING CARRIED OUT AT A TEMPERATURE OF ABOUT 165-170* C.,. A PRESSURE OF 120130 POUNDS/SQ. IN. GUAGE, AND FOR A PERIOD OF ABOUT 4 HOURS, RECOVERING AMMONIA AND SULFUR VALUES FROM SAID DIGESTION LIQUOR, SUBJECTING SAID CHIPS AFTER SAID DIGESTION PERIOD TO AN AQUEOUS TREATMENT AT A TEMPERATURE NOT SUBSTANTIALLY ABOVE SAID DIGESTION TEMPERATURE, REFINING SAID CHIPS BY MECHANICAL ACTION AFTER SAID AQUEOUS TREATMENT UNTIL SAID CHIPS ARE REDUCED TO A FIBROUS PULP, AND TREATING SAID PULP WITH A BLEACHING AGENT. 